Rechargeable lithium-ion cells can exhibit excellent charge-discharge cycle life, little or no memory effect, and high specific and volumetric energy. However, lithium-ion cells typically exhibit an inability to tolerate recharging to potentials above the manufacturer's recommended end of charge potential without degradation in cycle life. Recharging to potentials above the manufacturer's recommended end of charge potential is typically described as overcharge. Overcharge generally occurs when a current is forced through the cells and the charge delivered exceeds the charge-storing capability of the cell. Overcharge of lithium-ion cells can lead to the chemical and electrochemical degradation of cell components, rapid temperature elevation, and can also trigger self-accelerating reactions in the cells.
To combat these problems, redox shuttles have been used. Redox shuttles are chemical compounds that are incorporated into lithium-ion cells for overcharge protection. Generally, the redox shuttle can be reversibly electrochemically oxidized at a potential slightly higher than the working potential of a positive electrode of the lithium-ion cell. Use of the redox shuttles allows lithium-ion cells to normally operate in a voltage range less than the redox potential of the redox shuttle. If the lithium-ion cells are charged to a level that exceeds their normal cell capacity (i.e., are “overcharged”), the voltage will increase to the redox potential of the redox shuttle first and activate a redox mechanism, which will proceed as the only active component to transfer the excessive charge through the lithium-ion cells while minimizing damage. Use of such a mechanism inhibits overcharging.
Research and development has identified various options for redox shuttles. However, identifying shuttle candidates having both a suitably high redox potential and a sufficient service life has proven difficult. Many redox shuttles tend to chemically degrade over time or impede normal cell operation, thereby becoming useless. Accordingly, there remains opportunity for improvement.